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JavaScript

Posted by Josefin Wallgren
Josefin Wallgren
909 Points

How do I make it work?

I have this code:

function hello(){

var age = prompt('How old are you?');

if (age < 20) {
  alert('You are younger than 20');

}if (age == 20) {
  alert('Nice');

}if (age > 20) {
  alert('You are older than 20');

}if (typeof age === "string"){
  alert('Type numbers')
}
}

hello();

It was meant to pop up "Type numbers" if you write something else than a number in the field, but it comes up "Type numbers" even if I type a number... What am I doing wrong?

5 Answers

Dino Paškvan
Dino Paškvan
Courses Plus Student 44,108 Points

This is where things get a bit complicated... Whatever you enter into a prompt is actually a String, even if you type numbers. When you do those comparisons, it gets coerced into a number so that the engine can evaluate the if statement.

You can confirm this by entering this into the JavaScript console:

"19" < 20; // true
"21" < 20; // false

What you should be checking is if the String that gets entered into the prompt can be parsed as a number.

You can do this with the isNaN() function. It will return true if the parameter passed to the function is not a number or a string which can be coerced into a number. That's what NaN stands for — Not a Number.

So, your final if statement would look like this:

if (isNaN(age)) {
   alert('Type numbers');
}
Josefin Wallgren
Josefin Wallgren
909 Points

Thanks for the code, now it works great! :)

Mike Rogers
Mike Rogers
5,280 Points
Mike Rogers
Mike Rogers
5,280 Points

As Andrew points out you could use Regex, but you could also just use an else if for example:

function hello(){

  var age = prompt('How old are you?');

  if (age < 20) {
    alert('You are younger than 20');

  }else if (age == 20) {
    alert('Nice');

  }else if (age > 20) {
    alert('You are older than 20');

  } else {
    alert('Type numbers')
  }
}

hello();
Josefin Wallgren
Josefin Wallgren
909 Points

That's awesome! Thanks!

Andrew McCormick
Andrew McCormick
17,730 Points
Andrew McCormick
Andrew McCormick
17,730 Points

i believe the prompt always returns a string. you probably should use Regex to check if the input only contains numbers rather than using typeof

Josefin Wallgren
Josefin Wallgren
909 Points

What is Regex?

Aleem Mohamed
PLUS
Aleem Mohamed
Courses Plus Student 7,011 Points
Aleem Mohamed
Aleem Mohamed
Courses Plus Student 7,011 Points

I think when it comes to JavaScript prompts whatever you type into the prompt gets returned as a string. Therefore, the way you've structured your code you're always going to evaluate (typeof age === "string") to be true even if you enter a number. I'm fairly new to the programming game so someone more versed can correct me if I'm wrong.

Josefin Wallgren
Josefin Wallgren
909 Points
Josefin Wallgren
Josefin Wallgren
909 Points

Thank you so much for the quick answers! It really helped me understand this! :-)